Fire Performance, Thermal Degradation and Combustion Products of Flame-Retardant Hybrid Epoxy Composites

The fire performance of hybrid epoxy composites depends on both the flame-retardant system and the reinforcement architecture. This study investigates the effects of different combinations of synthetic and natural fabrics and functional fillers on the fire behavior, thermal degradation, smoke production, and combustion products of epoxy composites containing 10 wt.% expandable graphite (EG) and 10 wt.% ammonium polyphosphate (APP). A glass-fiber-reinforced composite was used as the reference. The materials were characterized by cone calorimetry, smoke density measurements, horizontal burning tests, thermogravimetric analysis (TG/DTG), differential scanning calorimetry (DSC), simultaneous thermal analysis coupled with Fourier-transform infrared spectroscopy (STA/FT-IR), and combustion-effluent analysis using a Purser furnace coupled with a Gasmet DX4000 gas analyzer. Significant differences in fire performance were observed among the formulations. VACMGBHF exhibited the longest time to ignition (206 s), the lowest peak heat release rate (79 kW·m−2), and the lowest maximum mass-loss rate during the principal thermo-oxidative degradation stage (2.03%·min−1). It also retained 38.8% of its initial mass at 900 °C, the highest value among the hybrid composites. In contrast, ACGBF showed the lowest average heat release rate (8 kW·m−2), maximum average rate of heat emission (20 kW·m−2), and total heat release (4 MJ·m−2). Smoke production also depended on composite composition. The lowest maximum specific optical density was recorded for the glass-fabric reference (42), whereas ACGBF exhibited the lowest total smoke release (123 m2·m−2). Visual examination revealed differences in the continuity and morphology of the post-combustion residues, with several hybrid formulations forming expanded, cohesive carbonaceous–mineral layers. TG/DTG, DSC, and STA/FT-IR analyses further demonstrated formulation-dependent degradation behavior and evolved-gas profiles. All composites achieved the HB classification in the horizontal burning test. The findings demonstrate that reinforcement architecture and filler composition influence heat release, smoke generation, and thermal stability in different ways, highlighting the need to evaluate these parameters together when designing flame-retardant epoxy composites.